Implantable active fixation lead with biodegradable helical tip

ABSTRACT

Described is an implantable lead comprising a flexible body extending between a proximal end and a distal end and a distal assembly coupled to the distal end of the body. The distal assembly includes a housing having a distal end and a proximal end, the proximal end fixedly coupled to the distal end of the lead body, a coupler rotatably disposed within the housing, the coupler having a proximal end and a distal end, and a helical electrode fixedly secured to the distal end of the coupler. The helical electrode comprises a proximal axial length portion that comprises a non-degradable material, and a distal axial length portion that comprises a biodegradable material. The coupler and the helical electrode are configured to rotate and therefore translate relative to the housing.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.13/295,445, entitled “IMPLANTABLE ACTIVE FIXATION LEAD WITHBIODEGRADABLE HELICAL TIP”, filed Nov. 14, 2011, which claims priorityto U.S. Provisional Application No. 61/432,812, entitled “IMPLANTABLEACTIVE FIXATION LEAD WITH BIODEGRADABLE HELICAL TIP”, filed on Jan. 14,2011, each of which is commonly owned and is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The present invention relates to implantable medical devices and relatesmore particularly to leads used with implantable medical devices forcardiac rhythm management (CRM) systems.

BACKGROUND

Various types of medical electrical leads for use in CRM andneurostimulation systems are known. For CRM systems, such leads aretypically extended intravascularly to an implantation location within oron a patient's heart, and thereafter coupled to a pulse generator orother implantable device for sensing cardiac electrical activity,delivering therapeutic stimuli, and the like. The leads frequentlyinclude features to facilitate securing the lead to heart tissue tomaintain the lead at its desired implantation site acutely, post-implantwhile normal processes of peri-tip wound healing and tissue ingrowthhave time to occur.

SUMMARY

In Example 1, an implantable lead including a flexible body extendingbetween a proximal end and a distal end, and a distal assembly coupledto the distal end of the body. The distal assembly includes a housinghaving a distal end and a proximal end, the proximal end fixedly coupledto the distal end of the lead body, a coupler rotatably disposed withinthe housing, the coupler having a proximal end and a distal end, and ahelical electrode fixedly secured to the distal end of the coupler. Thehelical electrode includes a proximal axial length portion thatcomprises a non-degradable material, and a distal axial length portionthat comprises a biodegradable material, wherein the coupler and thehelical electrode are configured to translate relative to the housingbetween fully extended and fully retracted positions.

In Example 2, the lead according to Example 1, wherein the helixassembly is configured such that the biodegradable portion and at leasta portion of the non-biodegradable portion of the helix extend beyondthe distal end of the housing when the helix assembly is extended.

In Example 3, the lead according to Example 1 or 2, wherein the distalaxial length portion comprises a sharp distal tip for penetratingtissue.

In Example 4, the lead according to Examples 1-3, wherein the proximalaxial length portion comprises a blunt distal end.

In Example 5, the lead according to Examples 1-4, wherein the helicalelectrode further comprises an interface between a distal end of theproximal axial length portion and a proximal end of the distal axiallength portion.

In Example 6, the lead according to Examples 1-5, wherein the distal endof the proximal axial length portion comprises a convex surface and theproximal end of the distal axial length portion comprises a concavesurface that is complementary to the convex surface.

In Example 7, the lead according to Examples 1-6, wherein the distal endof the proximal axial length portion comprises a plurality of threadsand the proximal end of the distal axial length portion comprises asurface that is complementary to the threads.

In Example 8, the lead according to Examples 1-7, wherein the proximalend of the distal axial length portion comprises a plurality of threadsand the distal end of the proximal axial length portion comprises asurface that is complementary to the threads.

In Example 9, the lead according to Examples 1-8, wherein the distal endof the proximal axial length portion comprises a male-shaped engagementmember and the proximal end of the distal axial length portion comprisesa female-shaped engagement member.

In Example 10, the lead according to Examples 1-9, wherein the proximalend of the distal axial length portion comprises a male-shapedengagement member and the distal end of the proximal axial lengthportion comprises a female-shaped engagement member.

In Example 11, the lead according to Examples 1-10, wherein at theinterface, the proximal axial length portion and the distal axial lengthportion are adhered to one another.

In Example 12, the lead according to Examples 1-11, wherein when thehelical electrode is in a fully extended position, a portion of thehelical electrode extends exterior and distal to the housing and has alength of about 0.025 inches to about 0.150 inches.

In Example 13, the lead according to Examples 1-12, wherein the distalaxial length portion of the helical electrode has a length of about0.005 inches to about half of length of the portion of the helicalelectrode that extends exterior and distal to the housing, and theproximal axial length portion has a length of about half of the lengthof the portion of the helical electrode that extends exterior and distalto the housing to about 0.145 inches.

In Example 14, the lead according to Examples 1-13, wherein the helicalelectrode comprises an outer diameter of about 0.025 inches to about0.075 inches.

In Example 15, the lead according to Examples 1-14, wherein the helicalelectrode comprises a wire comprising a cross-sectional diameter ofabout 0.005 inches to about 0.020 inches.

In Example 16, the lead according to Examples 1-15, wherein either oneor both of the proximal axial length portion and the distal axial lengthportion is electrically conductive.

In Example 17, a method of making an electrically active helix for alead, including the steps of: a) forming an elongated helix bodyprecursor having a proximal end and a distal end, said helix bodyprecursor comprises a proximal axial length portion comprising anon-degradable material and a distal axial length portion comprising abiodegradable material; and b) forming said helix body precursor into ahelix body in which a plurality of helical revolutions are formedbetween said proximal end and said distal end of said helix bodyprecursor.

In Example 18, a method of making a fixation helix for a lead,comprising the steps of: a) forming a first helical body having aproximal and a distal end between which a plurality of helicalrevolutions are formed, wherein the first helical body comprises anon-degradable material; b) forming a second helical body having aproximal and a distal end between which a plurality of helicalrevolutions are formed, wherein the second helical body comprises abiodegradable material; and c) joining the proximal end of the secondhelical body to the distal end of the first helical body.

In Example 19, the method of Example 18, wherein the steps of formingthe first and second helical bodies include forming the distal end ofthe first helical body and the proximal end of the second helical bodyto have complementary surfaces.

In Example 20, the method of Examples 18-19, wherein the step of formingthe first helical body includes forming a blunt distal end, and the stepof forming the second helical body includes forming a sharp distal tipfor penetrating tissue.

In Example 21, a method of making an electrically active helix for alead, comprising the steps of: a) forming an elongated tube ofbiodegradable material having first and second ends; b) forming anelongated tube of non-degradable material having first and second ends;c) attaching the first end of the biodegradable tube to the first end ofthe non-degradable tube to form a combined tube; and d) cutting thecombined tube into the shape of a helix.

While multiple embodiments are disclosed, still other embodiments of thepresent invention will become apparent to those skilled in the art fromthe following detailed description, which shows and describesillustrative embodiments of the invention. Accordingly, the drawings anddetailed description are to be regarded as illustrative in nature andnot restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a combined cutaway of a heart and a perspective view of animplantable medical device and lead in accordance with one embodiment.

FIG. 2 is a side elevation view of the lead of FIG. 1.

FIG. 3 is a partial cross-sectional view of the lead of FIG. 1 at E inFIG. 2 and shown in an extended position.

FIGS. 4A-4D are alternative enlarged, cross-sectional views of the leadof FIG. 3 at F.

FIG. 5 shows two steps in a method of making a fixation helix that maybe included in the lead of FIG. 1.

While the invention is amenable to various modifications and alternativeforms, specific embodiments have been shown by way of example in thedrawings and are described in detail herein. The intention, however, isnot to limit the invention to the particular embodiments described. Onthe contrary, the invention is intended to cover all modifications,equivalents, and alternatives falling within the scope of the inventionas defined by the appended claims.

DETAILED DESCRIPTION

FIG. 1 is a perspective view of an implantable medical device (IMD) 10.The IMD 10 includes a pulse generator 12 and a cardiac lead 14. The lead14 operates to convey electrical signals between the heart 16 and thepulse generator 12. The lead 14 has a proximal region 18 and a distalregion 20. The lead 14 includes a lead body, or flexible body, 22extending from the proximal region 18 to the distal region 20. Theproximal region 18 is coupled to the pulse generator 12 and the distalregion 20 is coupled to the heart 16. The distal region 20 includes anextendable/retractable fixation helix 24. The fixation helix 24 locatesand/or secures the distal region 20 of the lead 14 within the heart 16.The fixation helix 24 comprises a distal portion and a proximal portion,wherein at least a portion of the distal portion comprises abiodegradable portion, which will be discussed in greater detailelsewhere. The biodegradable portion may degrade or be resorbed after aperiod of time has passed after implantation of the lead 14 within theheart 16.

The pulse generator 12 typically includes a connector header 13 thatcouples the pulse generator 12 to the lead 14. The connector header 13typically contains one or more bores 17 that is/are able to receive aconnector(s) (not shown) that is/are part of a connector assembly (notshown, but see 40 in FIG. 2, discussed herein) formed near the proximalregion 18 of the lead 14, wherein electrical contacts (not shown) of theheader 13 couple with lead contacts (not shown) of the connectorassembly (not shown).

The header 13 is attached to a hermetically sealed enclosure 15 thatcontains a battery, electronic circuitry, and other components known tothose skilled in the art. Electrical contacts (not shown) in the header13 are any type known to those skilled in the art that are electricallyconnected via feedthroughs (not shown) mounted to extend through thehermetically sealed enclosure 15 in order to electrically couple thelead 14 with pulse generator 12.

The pulse generator 12 is typically implanted subcutaneously within animplantation location or pocket in the patient's chest or abdomen. Thepulse generator 12 may be any implantable medical device known in theart or later developed, for delivering an electrical therapeuticstimulus to a patient. In various embodiments, the pulse generator 12 isa pacemaker, an implantable cardioverter/defibrillator (ICD), a cardiacresynchronization (CRT) device configured for bi-ventricular pacing,and/or includes combinations of pacing, CRT, and defibrillationcapabilities.

The lead body 22 of the lead 14 can be made from any flexible,biocompatible material suitable for lead construction. In variousembodiments, the lead body 22 is made from a flexible, electricallyinsulative material. In one embodiment, the lead body 22 is made fromsilicone rubber. In another embodiment, the lead body 22 is made frompolyurethane. In various embodiments, respective segments of the leadbody 22 are made from different materials, so as to tailor the lead body22 characteristics to its intended clinical and operating environments.In various embodiments, proximal and distal ends of the lead body 22 aremade from different materials selected to provide desiredfunctionalities.

As is known in the art, the heart 16 includes a right atrium 26, a rightventricle 28, a left atrium 30 and a left ventricle 32. It can be seenthat the heart 16 includes a thin inner lining tissue of endocardium 34covering the thicker, muscular myocardium 36. In some embodiments asillustrated, the fixation helix 24, located at the distal region 20 ofthe lead, penetrates through the endocardium 34, and is imbedded withinthe myocardium 36. In one embodiment, the IMD 10 includes a plurality ofleads 14. For example, it may include a first lead 14 adapted to conveyelectrical signals between the pulse generator 12 and the rightventricle 28, and a second lead (not shown) adapted to convey electricalsignals between the pulse generator 12 and the right atrium 26.Additional leads, e.g., one or more coronary venous leads (not shown)for stimulating a left atrium 30 and/or a left ventricle 32, may also beemployed.

In the illustrated embodiment shown in FIG. 1, the fixation helix 24penetrates the endocardium 34 of the right ventricle 28 and is imbeddedin the myocardium 36 of the heart 16. In some embodiments, the fixationhelix 24 is electrically active and thus can be used to sense theelectrical activity of the heart 16 or to apply a stimulating pulse tothe right ventricle 28. This type of fixation helix 24 is also known asa “tip electrode,” and is generally a low-voltage electrode. In otherembodiments, the fixation helix 24 is not electrically active. Rather,in some embodiments, other components of the lead 14 are electricallyactive.

FIG. 2 is an isometric illustration of an embodiment of the lead 14. Aconnector assembly 40 is disposed at or near the proximal region 18 ofthe lead 14. The connector assembly 40 includes a connector 46 and aterminal pin 48. The connector 46 is configured to be coupled to thelead body 22 and is configured to mechanically and electrically couplethe lead 14 to the header 13 on the pulse generator 12 (see FIG. 1). Invarious embodiments, the terminal pin 48 extends proximally from theconnector 46 and in some embodiments is coupled to a conductor member(not visible in this view) that extends longitudinally through the leadbody 22 such that rotating the terminal pin 48 (relative to the leadbody 22) causes the conductor member to rotate within the lead body 22.In some embodiments, the terminal pin 48 includes an aperture (notshown) extending therethrough in order to accommodate a guide wire or aninsertion stylet.

A distal assembly 42 is disposed at or near the distal region 20 ordistal end of the lead 14 or lead body 22. Depending on the functionalrequirements of the IMD 10 (see FIG. 1) and the therapeutic needs of apatient, the distal region 20 of the lead 14 may include one or moreelectrodes. In the illustrated embodiment, the distal region 20 includesa single coil electrode, otherwise referred to as a “high-voltageelectrode,” or a pair of such coil electrodes 44 and 45, as shown, thatcan function as shocking electrodes for providing a defibrillation shockto the heart 16. In some embodiments, coil electrodes 44 and 45 includea coating that is configured to control (i.e., promote or discourage)tissue ingrowth. In various embodiments, the lead 14 may include only asingle coil electrode. In various other embodiments, the lead 14 alsoincludes one or more low-voltage electrodes (e.g., ring electrodes),such as electrode 43, along the lead body 22 in lieu of or in additionto the coil electrodes 44, 45. When present, the low-voltage electrodesoperate as relatively low-voltage, pace/sense electrodes. As will beappreciated by those skilled in the art, a wide range of electrodecombinations may be incorporated into the lead 14 within the scope ofthe various embodiments.

In some embodiments, and as shown in FIG. 2, the distal region 20 of thelead 14 may include an additional low-voltage (e.g., ring) electrode 43between the distal-most shocking coil 44 and the distal assembly 42.Such a lead is a tri-polar lead because it is configured with a tipelectrode (i.e., the fixation helix 24), a ring electrode 43 and a coilelectrode 44 (actually includes two coil electrodes 44, 45), or isotherwise known as a “dedicated bipolar lead.”

The distal assembly 42 includes a housing 50, within which the fixationhelix 24, or helical electrode, is at least partially disposed. As willbe explained in greater detail herein, the housing 50 accommodates amechanism that enables the fixation helix 24 to move distally andproximally relative to the housing 50, and that structure (not seen inthis view) may limit distal travel of the fixation helix 24 (relative tothe housing 50) in order to reduce or prevent over-extension of thefixation helix 24. As noted above, the fixation helix 24 operates as ananchoring means for anchoring the distal region 20 of the lead 14 withinthe heart 16.

In some embodiments, the fixation helix 24, or helical electrode, iselectrically active, and is used as a low-voltage, pace/sense electrode.In some embodiments, the fixation helix 24 is made at least in part ofan electrically conductive material such as ELGILOY™, MP35N™, tungsten,tantalum, iridium, platinum, titanium, palladium, stainless steel aswell as alloys of any of these materials. In some embodiments, thefixation helix 24 is made at least in part of a non-electricallyconductive material such as polyethersulfone (PES), polyurethane-basedthermoplastics, ceramics, polyetheretherketone (PEEK) and polypropylene.

FIG. 3 illustrates a partial cross-section of a portion of the lead 14.In FIG. 3, the fixation helix 24 is illustrated in an extended position.As shown, in the extended position, the helix assembly is configured sothat a distal, biodegradable portion 29 and at least part of a proximal,non-biodegradable portion 31 of the helix 24 extend beyond the distaltip 52 of the housing 50 (i.e., the distal extremity of the lead 14)when fully extended. In the illustrated embodiment, the fixation helix24 is electrically active so as to be operable as a low-voltage,pace/sense electrode.

As shown in FIG. 3, the housing 50 includes a distal region 52 and aproximal region 54. The proximal region 54 of the housing 50, in oneembodiment, is fixedly coupled to the distal end 20 of the lead body 22(FIG. 2). In various embodiments, the housing 50 is generally rigid orsemi-rigid. In some embodiments, the housing 50 is made of anelectrically conductive material such as ELGILOY™, MP35N™, tungsten,tantalum, iridium, platinum, titanium, palladium, stainless steel aswell as alloys of any of these materials. In some embodiments, thehousing 50 is made of a non-electrically conductive material such asPES, polyurethane-based thermoplastics, ceramics, polypropylene andPEEK.

As shown, the distal assembly 42 includes a coupler 58. In someembodiments, the coupler 58 is formed of a metallic material and isdisposed within the housing 50 and configured to move longitudinallyand/or rotationally with respect to the housing 50. The coupler 58includes a distal portion 60 and a proximal portion 64. In someembodiments, the proximal portion 64 of the coupler 58 is connected to aconductor member (not shown). The fixation helix 24 is connected to thedistal portion 60 of the coupler 58.

The fixation helix 24 has a distal region 71 and a proximal region 72.As shown in the embodiment of FIG. 3, the proximal region 72 is securedto the distal portion 60 of the coupler 58. One or more attachmentmethods are used to secure the fixation helix 24 to the coupler 58. Insome embodiments, the proximal region 72 of the fixation helix 24 iswelded or soldered onto the distal portion 60 of the coupler 58. In someembodiments, the proximal region 72 of the fixation helix 24 has aninner diameter that is less than an outer diameter of the distal portion60 of the coupler 58, and thus is held in place via compressive forces.In some embodiments multiple attachment methods are used.

In various embodiments, a conductor member (e.g., a conductor coil, notshown) is secured to the proximal portion 64 of the coupler 58, andextends proximally through the lead body 22 and the connector assembly40 (see FIG. 2). In such embodiments, the coupler 58 provides anelectrical connection between the conductor member and the fixationhelix 24. In the connector assembly 40, the conductor member is coupledto the terminal pin 48 (see FIG. 2) such that rotation of the terminalpin 48 causes the conductor member, the coupler 58 and the fixationhelix 24 to rotate. In other embodiments, the fixation helix 24 isrotated via a stylet that is inserted through an aperture that may beformed within the terminal pin 48. The distal assembly 42 is configuredso that relative rotation of the coupler 58 and fixation helix 24relative to the housing 50 results in longitudinal translation of thecoupler 58 and fixation helix 24 relative to the housing 50, therebyproviding the extendable/retractable functionality of the fixation helix24. It is emphasized, however, that the particular arrangementillustrated for facilitating extension and retraction of the fixationhelix 24 is exemplary only. In other words, any arrangement, whether nowknown or later developed, for providing the extendable/retractablefunctionality of the fixation helix 24 can be utilized in connectionwith the various embodiments.

FIG. 3 illustrates the distal assembly 42 of the present invention,which includes the fixation helix 24 having the distal biodegradableportion 29, at least a portion of which comprises a biodegradablematerial. The term “biodegradable” material is meant to includematerials capable of being degraded by hydrolytic or enzymatic reactionstypical to the anatomic site, as well as bioresorbable materials thatcan be broken down by the body and that do not require mechanicalremoval. The biodegradable portion 29 may be degraded by enzymaticprocesses or hydrolytic processes, for example. The biodegradableportion 29 may comprise any suitable biodegradable or bioresorbablematerial, including polymers and/or metals. Exemplary suitable materialsinclude magnesium alloys, polyglycolide, polylactide, and polydioxanone.The biodegradable or bioresorbable material may have various degradationor resorption rates depending on the desired rate for use in lead 14. Invarious embodiments, the degradation of the biodegradable portion 29takes place about one to two weeks after implantation, but otherdurations of degradation are also contemplated. The biodegradable orbioresorbable portion 29 may further comprise a therapeutic material ordrug that elutes over time having a desirable effect on surroundingcardiac tissues. The therapeutic material may be, for example, a steroidor other anti-inflammatory agent.

The remainder of the fixation helix 24, i.e., the non-degradable portion31, comprises a non-biodegradable or non-bioresorbable material that maybe chosen from materials commonly used to form conventional fixationhelixes or helical electrodes for implantable leads, such as thosedescribed herein, or another suitable material. As shown in FIG. 3, theembodiment allows for the biodegradable portion 29 and at least aportion of the non-degradable portion 31 to extend outside or distal tothe housing 50 of the distal assembly 42.

The biodegradable portion 29, in the embodiment shown in FIG. 3,comprises a tissue-piercing or sharp tip 35 at its distal-most point.The sharp tip 35 and length of coil is desired for immediate fixationpurposes, and to be able to pierce tissue during implantation. However,a sharp tip may irritate tissue (e.g., myocardium, epicardium,pericardium) and may cause inflammation and increase sensing andstimulating thresholds.

According to the various embodiments, the sharp tip 35, after a givenamount of time after implantation, may then degrade, or be resorbed ordissolved leaving behind a less sharp end portion (e.g., a blunt endportion), which may minimize trauma to tissue that could result from themotion of an implanted, sharp helix tip during dynamic cardiac cycles.This could reduce chronic tissue irritation, and decrease thresholdincreases caused by tissue trauma or irritation. The less sharp, orblunt, end portion may also reduce possible acute penetrations orperforations into the pericardial space and possible lead body migrationthrough the heart wall.

The biodegradable portion 29 may be attached or connected to theremainder of the fixation helix 24 (including the more proximalnon-degradable portion 31) at an interface or junction 33. The interfaceor junction 33 may be formed by any known or future developedtechniques. The technique chosen may depend upon the materials chosen tocomprise the portions 29, 31 of the fixation helix 24. Some exemplarymethods of joining portions 29 and 31 include, but are not limited to,welding, overmolding, mechanical joining, chemical joining or aMicro-Electro-Mechanical Systems (MEMS) technique.

In FIG. 3, dimensions of the fixation helix 24 and its components orparts are indicated with letters A, B, C, D and L. L indicates thelongitudinal length of fixation helix 24 that extends outside of, orexterior to, the housing 51 when the fixation helix 24 is in its fullyextended orientation, position, or configuration. Length L may rangefrom about 0.025 inches (0.635 mm) to about 0.150 inches (3.81 mm).Lengths A and B together make up length L. Length A corresponds to thelongitudinal length of the biodegradable portion 29 of the fixationhelix 24, and may range from about 0.005 inches (0.127 mm) to about halfthe length of L. Length B corresponds to the longitudinal length of thenon-degradable portion 33 that remains after the biodegradable portion35 degrades, and may range from about half the length of L to about0.145 inches (3.68 mm). Length B will provide a sufficient number ofturns of the fixation helix 24 in order to maintain the lead 14 in theheart tissue after the biodegradable portion 29 has degraded. Distance Cindicates the diameter of the fixation helix 24, and may range fromabout 0.025 inches (0.635 mm) to about 0.075 inches (1.91 mm). DistanceD is diameter of the wire or material that is coiled to form the helix24, and may range from about 0.005 inches (0.127 mm) to about 0.020inches (0.508 mm).

The aforementioned lengths provided above are exemplary only. Dependingupon the desired location of the fixation helix 24, such as an atrium ora ventricle, or the patient's unique cardiac pathophysiology, thevarious values for the dimensions A, B, C, D and/or L may vary fromthose specifically provided herein.

There are various embodiments of the interface 33 between thebiodegradable portion 29 and the non-degradable portion 31. Theembodiment of interface or junction 33 used results in certain shapes ofdistal ends of the non-biodegradable portion 31 that are left in placeafter degradation of the biodegradable portion 29.

FIG. 4A illustrates a cross-section of part of fixation helix 24indicated by F in FIG. 3. The junction 33 includes a convex, rounded endon the non-degradable portion 31, and a complementary concave end on thebiodegradable portion 29. After degradation of the biodegradable portion29, a convex, rounded distal end of the fixation helix 24 is leftbehind. The junction 33 may be formed between the biodegradable portion29 and non-degradable portion 31 in this embodiment using, for example,welding techniques.

FIG. 4B illustrates another possible cross-section of fixation helix 24indicated by F in FIG. 3. The junction 33 includes a generallymale-shaped non-degradable portion 31 that includes threads 35, and acomplementary female-shaped biodegradable portion 29. After degradationof the biodegradable portion 29, therefore, a threaded distal end of thefixation helix 24 is left behind. The threads 35 left behind on thenon-degradable portion 31 may advantageously serve to attach to hearttissue.

The embodiment shown in FIG. 4B may be made in various ways. Forexample, the biodegradable portion 29 and the non-degradable portion 31may be made separately and then threaded together. The threads 35, inthe embodiment, hold the two portions 29, 31 together. An alternativemethod of making the embodiment shown in FIG. 4B would be to overmoldthe biodegradable portion 29 onto the non-degradable portion 31. In suchan embodiment, for example, relief between threads 35 and non-threadedmaterial could serve to lock or attach the portions 29, 31 together.

FIG. 4C illustrates another possible cross-section of fixation helix 24indicated by F in FIG. 3. The junction 33 includes a male-shapedbiodegradable portion 29 that includes threads 35, and a complementaryfemale-shaped non-degradable portion 31. After degradation of thebiodegradable portion 29, therefore a female-shaped distal end withinterior threads 35 is left behind.

FIG. 4D illustrates yet another possible cross-section of fixation helix24 indicated by F in FIG. 3. The junction 33 includes a female-shapednon-degradable portion 31, and a complementary male-shaped biodegradableportion 29. The non-degradable and biodegradable portions 29, 31 arebonded or attached using, for example, adhesive along the junction 33.After degradation of the biodegradable portion 29, the degradableportion 31 is left behind.

Several methods may be used to make the fixation helix 24 of the lead ofthe present application. In one embodiment, the biodegradable portion 29and the remainder of the fixation helix 24 could be joined in wire formbefore the helix 24 is wound, resulting in helix 24. In anotherembodiment, as shown in FIG. 5, a tube 100 made of a biodegradablematerial portion 129 could be welded at a weld 133 to a non-degradablematerial portion 131. The tube 100 could then be laser cut (laser cutprocess indicated by arrow 200) to form fixation helix 24. The sharp tip35 could be cut after the portions 29, 31 are laser cut from tube 100.In yet another embodiment, the biodegradable portion 29 and thenon-degradable portion 31 could be formed separately having a helicalshape, and then could be attached afterwards.

Various modifications and additions can be made to the exemplaryembodiments discussed without departing from the scope of the presentinvention. For example, while the embodiments described above refer toparticular features, the scope of this invention also includesembodiments having different combinations of features and embodimentsthat do not include all of the described features. Accordingly, thescope of the present invention is intended to embrace all suchalternatives, modifications, and variations as fall within the scope ofthe claims, together with all equivalents thereof.

We claim:
 1. An implantable medical device comprising: a housing havinga distal end and a proximal end; a coupler rotatably disposed within thehousing, the coupler having a proximal end and a distal end; and ahelical electrode fixedly secured to the distal end of the coupler andconfigured with the coupler to translate relative to the housing betweenfully extended and fully retracted positions, the helical electrodecomprising: a proximal axial length portion that comprises anon-degradable material having a first surface; a distal axial lengthportion that comprises a biodegradable material having a second surfacethat is complementary to the first surface; and an interface between thebiodegradable material and the non-degradable material including thefirst surface and the second surface.
 2. The implantable medical deviceof claim 1, wherein the helical electrode is configured to senseelectrical activity of a patient's heart.
 3. The implantable medicaldevice of claim 1, wherein the helical electrode is configured to applya stimulating pulse to a patient's heart.
 4. The implantable medicaldevice of claim 1, wherein the helical electrode is configured such thatthe biodegradable portion extends beyond the distal end of the housingwhen the helical electrode is fully extended.
 5. The implantable medicaldevice of claim 4, wherein the helical electrode is configured such atleast a portion of the non-biodegradable portion of the helix extendsbeyond the distal end of the housing when the helical electrode isextended.
 6. The implantable medical device of claim 4, wherein theportion of the helical electrode that extends exterior and distal to thehousing, when the helical electrode is fully extended, has a length ofabout 0.025 inches to about 0.150 inches.
 7. The implantable medicaldevice of claim 6, wherein the distal axial length portion of thehelical electrode has a length of about 0.005 inches to about half oflength of the portion of the helical electrode that extends exterior anddistal to the housing, and the proximal axial length portion has alength of about half of the length of the portion of the helicalelectrode that extends exterior and distal to the housing to about 0.145inches.
 8. The implantable medical device of claim 1, wherein the distalaxial length portion comprises a portion configured to penetrate tissue.9. The implantable medical device of claim 8, wherein the portionconfigured to penetrate tissue comprises a sharp distal tip forpenetrating tissue.
 10. The implantable medical device of claim 8,wherein the portion configured to penetrate tissue comprises a bluntdistal end.
 11. The implantable medical device of claim 1, wherein thesecond surface comprises a convex surface and the first surfacecomprises a concave surface that is complementary to the convex surface.12. The implantable medical device of claim 1, wherein either one orboth of the proximal axial length portion and the distal axial lengthportion is electrically conductive.
 13. The implantable medical deviceof claim 1, wherein the either one or both of the proximal axial lengthportion and the distal axial length portion comprises an electricallyconductive material including at least one of ELGILOY™, MP35N™,tungsten, tantalum, iridium, platinum, titanium, palladium, andstainless steel.
 14. The implantable medical device of claim 1, whereinthe housing comprises a non-electrically conductive material.
 15. Animplantable medical device comprising: a housing having a distal end anda proximal end; a coupler rotatably disposed within the housing, thecoupler having a proximal end and a distal end; and a helical electrodefixedly secured to the distal end of the coupler and configured with thecoupler to translate relative to the housing between fully extended andfully retracted positions, the helical electrode comprising: a proximalaxial length portion that comprises a non-degradable material; and adistal axial length portion that comprises a biodegradable material; andwherein one of the distal end of the proximal axial length portion andthe proximal end of the distal axial length portion comprises aplurality of threads, and the other of the distal end of the proximalaxial length portion and the proximal end of the distal axial lengthportion comprises a surface that is complementary to the threads
 16. Theimplantable medical device of claim 15, wherein the distal end of theproximal axial length portion comprises a plurality of threads and theproximal end of the distal axial length portion comprises a surface thatis complementary to the threads.
 17. The implantable medical device ofclaim 15, wherein the proximal end of the distal axial length portioncomprises a plurality of threads and the distal end of the proximalaxial length portion comprises a surface that is complementary to thethreads.
 18. An implantable medical device comprising: a housing havinga distal end and a proximal end; a coupler rotatably disposed within thehousing, the coupler having a proximal end and a distal end; and ahelical electrode fixedly secured to the distal end of the coupler andconfigured with the coupler to translate relative to the housing betweenfully extended and fully retracted positions, the helical electrodecomprising: a proximal axial length portion that comprises anon-degradable material; and a distal axial length portion thatcomprises a biodegradable material; and wherein one of the distal end ofthe proximal axial length portion and the proximal end of the distalaxial length portion comprises a male-shaped engagement member, and theother of the distal end of the proximal axial length portion and theproximal end of the distal axial length portion comprises afemale-shaped engagement member.
 19. The implantable medical device ofclaim 18, wherein the distal end of the proximal axial length portioncomprises a male-shaped engagement member and the proximal end of thedistal axial length portion comprises a female-shaped engagement member.20. The implantable medical device of claim 18, wherein the proximal endof the distal axial length portion comprises a male-shaped engagementmember and the distal end of the proximal axial length portion comprisesa female-shaped engagement member.